Optical interference technology has provided multilayer thin film structures and coatings for controlling reflectance, transmittance and absorptance of light energy, for example, thin film multilayer coatings that are infrared (IR) reflective and radio frequency (RF) transparent, signature control films that are reflective only in particular bands of the infrared and are otherwise transmissive or absorptive, coatings for modifying the spectral emittance of a surface by changing its reflectance and absorptance, and dichromatic color-shift coating compositions, to name but a few.
For convenience and ease of application of these thin film optical structures to surfaces and objects intended to be coated thereby, it is desirable to provide the same in the form of brushable or sprayable liquid coating compositions for application in much the same manner as paint. For such purposes, the thin film structures, if in flat or flake form, must be two-sided and produced in or reduced to a performance size in order to be suitable for use as pigments in a liquid film forming binder.
By way of example, U.S. Pat. No. 4,434,010 to Ash, and U.S. Pat. Nos. 5,059,245, 5,569,535 and 5,571,624 to Phillips et al., disclose various thin film multilayer optical structures suitable for use in a variety of optical coating compositions, namely, (1) an optical coating comprised of alternating layers of dielectric materials having different indices of refraction which, when transformed into optical paint chips or flakes, can be utilized to produce hot mirrors, cold mirrors, other selective reflectors, and optically variable color shift paints; (2) an optical coating structure comprised of cermet layers deposited on opposite sides of a central layer of reflective material and comprising pigments for use in paints having properties of selective solar absorption; and (3) an optical coating comprised in sequence of a semi-opaque metal layer, a dielectric layer, a metal reflecting layer, another layer of dielectric material and a final semi-opaque metal layer which produces strongly dichroic optical effects and finds particular usefulness in optically variable automotive body paints. U.S. Pat. No. 5,569,535 refers to the paint particles as high chroma color-shifting interference thin film platelets. U.S. Pat. No. 5,571,624 illustrates the manner in which the platelets are used in automotive body paints.
Since the platelets or flakes may be oriented either right side up or upside down in the final paint film, the flakes must be of symmetrical construction and exhibit the same optical properties on each of their two sides, as is illustrated in said patents.
U.S. Pat. No. 4,838,648, also to Phillips et al., discloses a variant wherein the optically variable platelets include a reflective metal layer that is magnetic, so that the paint can be magnetically encoded as well as providing color shift.
Heretofore, the flakes of multilayer thin film materials have been produced by precoating a carrier substrate with a soluble release coating and depositing thin films of the optical materials in preselected sequence onto the carrier to build up the desired multilayer thin film structure. Alternatively, the substrate itself may be soluble. After deposition of the multilayer structure, the release coat (or the substrate) is dissolved, thereby freeing the multilayer structure from the substrate so that the same can be broken up into small flakes. To insure that the flakes are of the requisite size for use as pigment particles, the same are sized by means of two screens or sieves, the first to screen out oversize particles and the second to screen out undersize particles. Oversize particles are further processed to pass through the first screen or sieve in order to serve as useful pigment particles. Undersize particles that pass through the second screen must be discarded as waste because they do not perform well and reduce the overall performance of the final coating composition. The process is tedious, time consuming and wasteful.
In an effort to make the process less wasteful and more efficient, it has been proposed to use a foraminous screen or a waffle pattern on the carrier substrate so that the multilayer film structure will be formed into pre-sized flakes, and thus divided into pre-sized flakes when it is released from the substrate. However, two sieve screening is still required.
Moreover, the pigment flakes thus produced are very thin, on the order of 0.1 to 1.0 microns. Consequently, many layers of flakes will be needed to produce a coating of given film thickness having the desired optical characteristics. For example, dichromatic coatings having a pigment particle or flake thickness of about 0.8 microns dispersed in a liquid binder or carrier at a particle volume concentration (PVC) of 15% and applied at a coating or paint film thickness of 0.004 inch (101.6 microns) will require 15.24 microns of thin film structure, which in turn dictates use of 19 of the flakes throughout the paint film thickness, even though it is only the top flake in the coating that provides the dichromatic effect, i.e., the visual color shift when the paint is viewed from different angles. As another example, for infrared reflectance, the concentration or loading of flakes in a film forming binder must be in the order of 10 to 40% by volume in order to achieve high reflectance. For a typical coating thickness of 50 microns, typical flake thickness of 0.2 microns, and a volume loading of 20%, fifty flakes are required through the thickness of the coating, despite the fact it is only the top flake that provides the optical benefit. This constitutes an extremely inefficient use of very expensive multilayer flakes.
U.S. Pat. No. 5,506,053 to Ronald N. Hubbard, discloses an infrared (IR) reflective, radio frequency (RF) transparent coating composition wherein the pigment particles comprise thin film coated dielectric microspheres ranging in size from about 5 to about 500 microns. The microspheres thus fill up a large volume in a coating composition so that only a few layers, e.g., one or two layers of the coated spheres are required in a coating of conventional thickness.